im a bit of a bird nerd here, and while they do put pack a lot in, its not really fat as you and i would think of it. 90% of what you see is just to floof to keep warm.

my b&g macaw will put on a little weight in fall even tho hes a tropical chicken. altho admitly that could just be me feeding him differently/different times, cause of the lack of light through the winter.